An off-route obstruction clearance altitude
(OROCA) is an off-route altitude that provides
obstruction clearance with a 1,000-foot buffer in nonmountainous
terrain areas and a 2,000-foot buffer in
designated mountainous areas within the U.S. This
altitude may not provide signal coverage from
ground-based navigational aids, air traffic control
radar, or communications coverage. OROCAs are
intended primarily as a pilot tool for emergencies and
situational awareness. OROCAs depicted on NACO
en route charts do not provide you with an acceptable
altitude for terrain and obstruction clearance for the
purposes of off-route, random RNAV direct flights in
either controlled or uncontrolled airspace. OROCAs
are not subject to the same scrutiny as MEAs, MVAs,
MOCAs, and other minimum IFR altitudes. Since
they do not undergo the same obstruction evaluation, airport airspace analysis procedures, or flight inspection,
they cannot provide the same level of confidence
as the other minimum IFR altitudes.

If you depart an airport VFR intending to or needing to
obtain an IFR clearance en route, you must be aware of
the position of your aircraft relative to terrain and
obstructions. When accepting a clearance below the
MEA, MIA, MVA, or the OROCA, you are responsible
for your own terrain/obstruction clearance until
reaching the MEA, MIA, or MVA. If you are unable
to visually maintain terrain/obstruction clearance, you
should advise the controller and state your intentions.
[Figure 3-34]

For all random RNAV flights, there needs to be at least
one waypoint in each ARTCC area through which you
intend to fly. One of the biggest problems in creating
an RNAV direct route is determining if the route goes
through special use airspace. For most direct routes,
the chances of going through prohibited, restricted,
or special use airspace are good. In the U.S., all direct
routes should be planned to avoid prohibited or
restricted airspace by at least 3 NM. If a bend in a
direct route is required to avoid special use airspace,
the turning point needs to be part of the flight plan.
Two of the most prominent long range navigation
systems today include FMS with integrated GPS
and stand-alone GPS. The following example is a
simplified overview showing how the RNAV systems
might be used to fly a random RNAV route:

In Figure 3-35 on page 3-30, you are northeast of
Tuba City VORTAC at FL 200 using RNAV (showing
both GPS and FMS), RNAV direct on a southwesterly
heading to Lindbergh Regional Airport in Winslow.
As you monitor your position and cross check your
avionics against the high altitude en route chart, you
receive a company message instructing you to divert
to Las Vegas, requiring a change in your flight plan as
highlighted on the depicted chart excerpt.

During your cockpit review of the high and low altitude
en route charts, you determine that your best course of
action is to fly direct to the MIRAJ waypoint, 28 DME
northeast of the Las Vegas VORTAC on the 045° radial.
This places you 193 NM out on a 259° magnetic course
inbound, and may help you avoid diverting north, allowing you to bypass the more distant originating
and intermediate fixes feeding into Las Vegas. You
request an RNAV random route clearance direct
MIRAJ to expedite your flight. Denver Center comes
back with the following amended flight plan and initial
clearance into Las Vegas:

“Marathon five sixty four, turn right heading two six
zero, descend and maintain one six thousand, cleared
present position direct MIRAJ.”
The latitude and longitude coordinates of your present
position on the high altitude chart are N36 19.10, and
W110 40.24 as you change your course. Notice your
GPS moving map (upper left) and the FMS control
display unit (below the GPS), and FMS map mode
navigation displays (to the right of the GPS) as you
reroute your flight to Las Vegas. For situational
awareness, you note that your altitude is well above
any of the OROCAs on your direct route as you arrive
in the Las Vegas area using the low altitude chart.